The present invention relates to position determination, particularly but not exclusively to a method and device for permitting a user to find a location which is not within the coverage of the Global Positioning System (GPS).
The Global Positioning System (GPS) is a well-known satellite based system which provides position information to GPS receivers, for example the commercially available Garmin(trademark) range of GPS receivers.
GPS is especially suitable for use in wide open areas, but, due to the nature of satellite communications, its availability is limited in highly built-up areas and underground, for example in an underground car park. To extend the apparent coverage of the GPS system from a user""s point of view, other techniques are used to interpolate during the periods when the user is out of range of the GPS system. Such interpolation is performed by systems which are commonly referred to as dead reckoning systems, which involve estimating the current location by tracking movements from the last known GPS position, using sensors such as gyros, accelerometers, compasses and so on.
FIG. 1 illustrates a conventional interpolation system. A vehicle 1 equipped with a GPS receiver 2 and a dead reckoning system 3 travels between areas with GPS coverage 4, 5 through an area without GPS coverage 6. The position 7 of the vehicle 1 just before it enters the area without GPS coverage 6 is known from GPS data. During its transit through the non-GPS area 6, the dead reckoning system 3 estimates the position of the vehicle 1 based on its initial position 7 and its subsequent movements, using direction and speed detectors. On re-entry into an area with GPS coverage 5, the GPS receiver 2 once again takes over to provide accurate positional data 8. Depending on the accuracy of the dead reckoning system 3, there may be a discrepancy between the position 8 of the vehicle given by the GPS system and the position 9 estimated by the dead reckoning system 3. A vehicle position detector which uses a combination of GPS receiver and dead reckoner is described in U.S. Pat. No. 5,257,195.
If a user switches on a GPS receiver in an area where no GPS signal is available, he receives no positional information and is therefore unable to fix his position. The present invention aims to address this problem.
In accordance with the present invention, there is provided a method of providing positional information to a receiver which is arranged to receive a positioning system signal, the signal providing information relating to the receiver""s current position, comprising the steps of tracking the receiver""s route from a first position at which the positioning system signal is not available to a second position at which the positioning system signal is available, determining positional information defining the first position from the positioning system signal at the second position and the receiver""s route between the first and second positions, and storing the positional information at the receiver.
The positional information defining the first position can be determined by back tracking from the second position along the route taken by the receiver between the first and second positions. By tracking the receiver""s route until a positioning system signal, for example a GPS signal, is obtained, the user can be provided with information relating to his starting position once he has reached the position at which the GPS signal is available. This enables the user to return to his starting position from any other known position.
According to the present invention, there is further provided a device for providing positional information to a user, the device being arranged to receive a positioning system signal, the signal providing information relating to the current position of the device, comprising means for tracking the route of the device from a first position at which the positioning system signal is not available to a second position at which the positioning system signal is available, means for determining positional information defining the first position from the positioning system signal at the second position and the receiver""s route between the first and second positions, and means for storing the positional information.